Interconnect structure including air gap

ABSTRACT

A method of forming a semiconductor structure is provided. A conductive layer is formed over a substrate. The conductive layer is selectively etched to form a first conductive portion, a second conductive portion, and a spacing between the first conductive portion and the second conductive portion. A dielectric layer is formed over the first conductive portion, the second conductive portion, and the spacing, such that an air gap is formed in the spacing between the first and second conductive portions and is sealed by the dielectric layer.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological progress in IC manufacture has produced severalgenerations of ICs, and each generation fabricates smaller and morecomplex circuits than the previous generation. Currently, thesemiconductor industry has progressed into nanometer technology nodesfor higher device density and better electrical performance. In thepast, the reductions of the feature size were limited by the ability todefine the structures photo-lithographically. Recently, devicegeometries having smaller dimensions created new challenges. Forexample, for two adjacent conductive lines, when the distance betweenthe conductive lines is decreased, the semiconductor devices suffer fromseveral electrical and processes issues. Conventional techniques havenot been entirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a flow chart illustrating a method of forming a semiconductorstructure according to some embodiments of the present disclosure.

FIG. 1B is a flow chart illustrating a method according to variousembodiments of the present disclosure.

FIGS. 2-13 are drawings schematically illustrating a method of forming asemiconductor structure in various process stages according to variousembodiment of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

The semiconductor industry has continually improved the speed and powerof integrated circuits (ICs) by reducing the size of components withinthe ICs. Several advanced techniques have been developed to implementtechnique nodes with smaller feature sizes. When the spacing between twoadjacent conductive lines is smaller than a certain dimension, thesemiconductor suffers from several issues such as RC delay, timedependent dielectric breakdown (TDDB), and parasitic capacitance. Toimprove these issues, an air gap is formed between two adjacentconductive lines as air has a low dielectric constant of 1. However,when the spacing between two adjacent conductive line is smaller than acertain dimension such as for example about 50 nm or less, conventionalprocesses in the back-end of line (BEOL) suffer from process problemsthat degrade device performance and decrease the manufacture yield.Accordingly, one of the aspects of the present disclosure is to providea solution to these problems.

The present disclosure relates generally to a semiconductor structureand a method of manufacturing the semiconductor structure. Inembodiments, the method disclosed herein may be applied in BEOL, forexample. According to various embodiments of the present disclosure, themethod disclosed herein may form an in-situ air gap, which isself-aligned, between two adjacent conductive lines. Various embodimentsof the present disclosure will be described in detail hereinafter.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the embodiments. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

FIG. 1A is a flow chart illustrating a method 10 of forming asemiconductor structure according to some embodiments of the presentdisclosure. The method 10 includes operation 11, operation 12, operation13, operation 14, operation 15, and operation 16. FIGS. 2-13collectively illustrate more detailed manufacturing methods as a seriesof cross-sectional views or plan views in accordance with someembodiments of the present disclosure. It will be appreciated thatalthough these methods each illustrate a number of operations, actsand/or features, not all of these operations, acts and/or features arenecessarily required, and other un-illustrated operations, acts and/orfeatures may also be present. Also, the ordering of the operationsand/or acts in some embodiments can vary from what is illustrated inthese figures. In addition, the illustrated acts can be further dividedinto sub-acts in some implementations, while in other implementationssome of the illustrated acts can be carried out concurrently with oneanother.

Referring to FIG. 1A, the method 10 includes an operation 11 of forminga first dielectric layer over a substrate. FIG. 2 is a cross-sectionalview schematically illustrating the formation of a first dielectriclayer 110 over a substrate 101 according to some embodiments of thepresent disclosure.

In some embodiments, the substrate 101 may include a bulk siliconsubstrate. In yet some embodiments, the substrate 101 include anelementary semiconductor such as for example germanium or silicon in acrystalline structure. In yet some embodiments, the substrate 101include a compound semiconductor such as for example gallium arsenic,silicon carbide, indium phosphide, gallium phosphide or indiumantimonide, or combinations thereof. In yet some embodiments, thesubstrate may include an alloy semiconductor such as silicon germanium,silicon germanium carbide, gallium arsenic phosphide, and gallium indiumphosphide. In some embodiments, the substrate 101 may include anintegrated circuit 103. The integrated circuit 103 may include activedevices such as transistors and the like. The substrate 101 may includedielectric layer and via contacts (not shown in FIG. 2) over theintegrated circuit 103. The via contacts may be fabricated over theintegrated circuit 103 such that the integrated circuit 103 may beelectrically connected with other devices. In some embodiments, thesubstrate 101 has experienced processes of the front-end of line and amiddle-end of line.

In some embodiments, the first dielectric layer 110 may include oxidessuch as silicon oxide, undoped silicate glass (USG), borophosphosilicateglass (BPSG), fluorinated silicate glass (FSG). In some embodiments, thefirst dielectric layer 110 may include low-k dielectrics such as carbondoped oxides, porous carbon doped silicon dioxide, a polymer such aspolyimide, the like, or a combination thereof. The low-k dielectricmaterials may have k values lower than 3.9. The first dielectric layer110 may be deposited by physical vapor deposition (PVD) processes,chemical vapor deposition (CVD) processes, atomic layer deposition (ALD)processes, a spin-on-dielectric (SOD) process, and the like.

In yet some embodiments, the first dielectric layer 110 includes a low-klayer 114 over the substrate 101 and an etching stop layer 116 on thelow-k layer 114. Illustrative examples of the material of the etchingstop layer 116 include TiN, SiN, Al_(x)O_(y)N_(z), nitrogen dopedsilicon carbide, the like, and a combination thereof. The etching stoplayer 116 may be formed by techniques such as for example PVD, CVD, SOD,ALD, and the like.

Referring to FIG. 1A, the method 10 proceeds to operation 12 by forminga via opening in the first dielectric layer. FIGS. 3-4 arecross-sectional view schematically illustrating the detailed steps offorming a via opening 112 in the first dielectric layer 110 according tosome embodiments of the present disclosure. In FIG. 3, a patternedmasking layer 117 with one or more opening 117 a are formed over thefirst dielectric layer 110. The patterned masking layer 117 may includea photoreist layer or a hard mask layer. The illustrated patternedmasking layer 117 may include a bottom layer, a middle layer, anantireflective coating (not shown), and/or the like. In FIG. 4, anetching process is performed to form one or more via openings 112 in thefirst dielectric layer 110. The etching process may includes wet etchingprocesses or dry etching process such as for example reactive ionetching techniques or plasma etching techniques. After forming the viaopening 112, the patterned masking layer may be removed according tosome embodiments of the present disclosure.

Referring to FIG. 1A and FIG. 5, the method 10 proceeds to operation 13by forming a barrier layer 120 lining the via opening 112 and coveringthe first dielectric layer 110. Illustrative examples of the material ofthe barrier layer 120 includes Ta, TaN, and Co, the like, and acombination thereof. The barrier layer 120 may prevent the metallicmaterial, which is formed in subsequent processes, from migration to thefirst dielectric layer 110 according to some embodiments of the presentdisclosure.

Referring still to FIG. 1A, the method 10 proceeds to operation 14 byforming a conductive layer overlaying the barrier layer and filling thevia opening. FIGS. 6-7 are cross-sectional view schematicallyillustrating the detailed steps of operation 14. In FIG. 6. a metallayer 130″ is deposed overlaying the barrier layer 120 and filling thevia opening 112. In FIG. 7, a planarization process such as for examplechemical mechanical polishing (CMP) process may be carried out on thetop of the metal layer 130″ so as to form the conductive layer 130. Theconductive layer 130 is formed overlaying the barrier layer 120 andfilling the via opening 112. Illustrative examples of conductive layer130 includes Cu, Al, W, Co, the like, and a combination thereof. Theconductive layer 130 includes a first portion 130 a filled in the viaopening 112. The first portion 130 a and the barrier layer 120 in thevia opening 112 collectively constitutes one or more via contacts 134for vertical interconnection. The conductive layer 130 further includesa second portion 130 b over the first dielectric layer 110 and thebarrier layer 120. The planarization process does not remove all of thesecond portion 130 b of the metal layer 130″, and most of the secondportion 130 b of the metal layer 130″ may be left. Accordingly, theplanarization process disclosed herein differs essentially from the CMPused in the damascene or dual damascene processes.

Turning back to FIG. 1A, the method 10 proceeds to operation 15 bypatterning the conductive layer and the barrier layer to form a firstconductive portion, a second conductive portion, and a spacing therebetween. FIGS. 8 and 9A are cross-sectional view schematicallyillustrating the detailed steps of operation 15 according to someembodiments of the present disclosure. As shown in FIG. 8, a maskinglayer 150 is formed over the conductive layer 130. The masking layer 150has a pattern with one or more openings 151 exposing portions of theconductive layer 130. In some embodiments, the masking layer 150 may bea single-layered structure or a multiple-layered structure including abottom layer, a middle layer, an antireflective coating, and/or thelike. Next, as shown in FIG. 9A, the conductive layer 130 and thebarrier layer 120 are patterned to form a first conductive portion 131,a second conductive portion 132, and a spacing 135 between the firstconductive portion 131 and second conductive portion 132. In someembodiments, pattering the conductive layer 130 and the barrier layer120 includes transferring the pattern of the masking layer 150 to theconductive layer 130 and the barrier layer 120. For example, patterningthe conductive layer 130 and the barrier layer 120 may includeselectively etching the conductive layer 130 and the barrier layer 120,thereby forming the first conductive portion 131, the second conductiveportion 132, and the spacing 135. In some embodiments, at least one ofthe first and second conductive portions 131, 132 is physicallyconnected to the via contacts 134. The spacing 135 may cut through bothof the conductive layer 130 and the barrier layer 120. Accordingly, theremained portions 120 r of the barrier layer 120 are under the firstconductive portion 131 and the second conductive portion 132. Inaddition, the spacing 135 reveals or exposes a portion of the firstdielectric layer 110. For example, the exposed portion of the firstdielectric layer 110 may be a portion 116 a of the etching stop layer116 of the first dielectric layer 110, as shown in FIG. 9A. In someembodiments, the selectively etching process may includes wet etchingprocesses or dry etching process such as for example plasma etchingtechniques or reactive ion etching (RIE) techniques. In examples, theselectively etching process employs RIE techniques using etchantincluding at least one of Cl₂, F₂, H₂, and C_(x)F_(y) such as CF₄, CHF₃,CH₂F₂, C₂HF₅, and C₂H₂F₄.

Referring to FIG. 9B, the masking layer 150 may be removed after theformation of the first conductive portion 131, the second conductiveportion 132, and the spacing 135, according to some embodiments of thepresent disclosure.

It various embodiments, the first and second conductive portions 131,132 are not completely formed or embedded in the trenches or vias of thefirst dielectric layer 110, and therefore the first and secondconductive portions 131, 132 are different from the damascene conductiveregions. In some embodiments, the top surfaces 131 t of the firstconductive portions 131 and the top surface 132 t of the secondconductive portions 132 extend at a substantially identical level thatis higher than a top surface 110 t of the first dielectric layer 110.Further, the sidewalls 131 s, 132 s of the first and second conductiveportions 131, 132 are exposed and constitutes the lateral surfaces ofthe spacing 135. Accordingly, the embodiments disclosed herein differsessentially from the damascene or dual damascene processes.

As shown in FIG. 9B, the spacing 135 may include an inlet 135 i having awidth W1 and a bottom 135 b having a width W2. The width W1 of the inlet135 i is greater than the width W2 of the bottom 135 b according to someembodiments of the present disclosure. In some examples, the width W1 ofthe inlet 135 i may be ranged from about 60 nm to about 5 nm, morespecifically from about 50 nm to about 5 nm. For example, the width W1may be about 5 nm, 7 nm, 10 nm, 20 nm, and 40 nm. In addition, the widthW2 of the bottom 135 b may be ranged from about 50 nm to about 3 nm,specifically from about 45 nm to about 5 nm, more specifically fromabout 40 nm to about 16 nm.

FIG. 9C illustrates a top view of FIG. 9B according to some embodimentsof the present disclosure, in which FIG. 9B is taken along line A-A′ inFIG. 9C. As shown in FIG. 9C, the first conductive portion 131 andsecond conductive portion 132 are conductive lines or wirings so thatthe first conductive portion 131 and second conductive portion 132 mayalso refer to a first conductive wiring 131 and second conductive wiring132 in some embodiments of the present disclosure. Further, the spacing135 is present between the first conductive portion 131 and secondconductive portion 132. In some embodiments, the first conductiveportion 131, the second conductive portion 132, and the spacing 135extend along a first direction D1. In examples, the first conductiveportion 131, the second conductive portion 132, and the spacing 135 maybe substantially parallel with each other.

Referring to FIG. 10, the method 10 may optionally include an act ofselectively etching the first dielectric layer 110 to form an aperture137 under the spacing 135, after forming the first conductive portion131, the second conductive portion 132, and the spacing 135. In someembodiments, selectively etching the first dielectric layer 110 includesremoving the exposed portion 116 a of the etching stop layer 116 of thefirst dielectric layer 110 using the first and second conductiveportions 131, 132 as a mask, thereby forming an aperture 137 under thespacing 135. In some examples, the aperture 137 has a width W3 less thanthe width W1 of the inlet 135 i of the spacing 135(shown in FIG. 9B). Inyet some examples, the width W3 of the aperture 137 is greater than thewidth W2 of the bottom 135 b of the spacing 135 (shown in FIG. 9B). Inyet some embodiments, the aperture 137 communicates with the spacing135. The spacing 135 and the aperture 137 collectively form a voidregion 136. The spacing 135 forms an upper portion of the void region136, and may also refer to as the upper portion 135 of the void region136. The aperture 137 forms a bottom portion of the void region 136, andmay also refer to as the bottom portion 137 of the void region 136. Insome examples, the void region 136 includes a neck portion 136 n, whichhas a minimum width W4 through out the void region 136. The ratio of thewidth W4 of the neck portion 136 n to the width W5 of the inlet 136 i ofthe void region 136 may be ranged from about 0.5 to about 0.95,specifically from 0.6 to 0.95, more specifically from 0.8 to 0.95. It isnoted that the width W5 of the inlet 136 i may be the same as ordifferent from the width W1 of the inlet 135 i of the spacing 135. Insome embodiments, the width W1 of the inlet 135 i is possibly changedduring selectively etching the etching the stop layer 116 of the firstdielectric layer 110. In embodiments, the selectively etching processmay includes wet etching processes or dry etching process such as forexample plasma etching techniques and/or reactive ion etching (RIE)techniques.

Referring to FIG. 11, after the formation of the aperture 137, themethod 10 may optionally include an act of forming a capping layer 138covering the first and second conductive portions 131, 132. In someembodiments, the capping layer 138 covers the top and sidewalls of eachof the first and second conductive portions 131, 132. Accordingly, thefirst and second conductive portions 131, 132 are encapsulated by thecapping layer 138 and the remained portions 120 r of the barrier layer120. In some examples, the capping layer 138 includes material such asfor example cobalt silicide (Co—Si_(x)), Co, Mn, Ru, the like, and acombination thereof. The capping layer 138 and the remained portions 120r of the barrier layer 120 may suppress or prevent the material of theconductive portions 131, 132 from migration to the dielectric formed inthe subsequent processes.

In yet some embodiments, the capping layer 138 may be formed prior tothe formation of the aperture 137. For example, the capping layer 138may be formed immediately after the patterning of the conductive layer130 and the barrier layer 120 depicted in FIG. 9B.

Referring back to FIG. 1A, the method 10 proceeds to operation 16 byforming a second dielectric layer covering the spacing and the first andsecond conductive portions to enclose a void gap in the spacing. FIG. 12A is a cross-sectional view illustrating the formation of a seconddielectric layer 140. The second dielectric layer 140 covers the firstand second conductive portions 131, 132 and the void region 136 (orspacing 135) there between. It is noted that the void region 136 is notfilled with the second dielectric layer 140 so that an in-situ formedvoid gap 142 is fabricated at the position of the void region 136between the first and second conductive portions 131, 132. The void gap142 may also referred to as an air gap 142. In some embodiments, thesecond dielectric layer 140 may be formed by non-conformal chemicalvapor deposition techniques, spin-on coating techniques, and the like.The void gap 142 may be formed under the second dielectric layer 140 byselecting suitable material for the deposited material and/or processparameters. Illustrative examples of suitable materials for the seconddielectric layer 140 includes, but not limited to, nitrogen dopedsilicon carbide, oxygen doped silicon carbide, TEOS, Si₃N₄, and thelow-k material, and the like. The air gap or void gap 142 with lowdielectric constant (e.g., 1) may improve various electricalinterferences between the first and second conductive portions 131, 132,such as the parasitic capacitance and RC delay of the conductivefeatures of back-end of line (BEOL). Further, electromigration (EM) andtime dependent dielectric breakdown (TDDB) of devices may be improved.

FIG. 12 B is a top view illustrating the structure of FIG. 12A, in whichFIG. 12 A is taken along line A-A′ in FIG. 12B. The void gap 142 isin-situ formed in position of the void region 136 (or spacing) betweenthe first and second conductive portions 131, 132. Furthermore, the voidregion 136 (or spacing) may be concurrently formed with the first andsecond conductive portions 131, 132 according to some embodiments of thepresent disclosure. Therefore, the formation of the air gap or void gap142 does not require an addition photolithographic process so that themethod disclosed herein is cost effective, according to some embodimentsof the present disclosure. Furthermore, in convention methods where theair gap is formed using an addition mask, it suffers misalignment issuesin advanced technical nodes. To the contrary, the embodiments disclosedherein advantageously involves an in-situ method of forming the air gapor void gap between the first and second conductive portions 131, 132,and therefore the misalignment issues in convention techniques areavoid.

After operation 16, the method 10 may include other operations or steps.FIG. 13 illustrates additional via contacts 144 formed in seconddielectric layer 140, and conductive lines 161, 162 formed over thesecond dielectric layer 140. The via contacts 144 are electricallycoupled to the respective underlying conductive portions 131, 132. Theconductive lines 161, 162 are disposed over and electrically coupled torespective underlying via contacts 144. A dielectric layer 170 is formedcovering the conductive lines 161, 162 and the second dielectric layer140. An air gap 164 may be formed the between conductive lines 161 and162 and between the dielectric layers 170 and 170. The formation processmay be essentially the same as in FIGS. 2 through 12A, and are notrepeated herein.

FIG. 1B is flowchart illustrating a method 20 according to variousembodiments of the present disclosure. The method 20 includes anoperation 21 of forming a conductive layer over a precursor substrate.FIG. 7 illustrates the formation of a conductive layer 130 over aprecursor substrate 200. In some embodiments, the precursor substrate200 may include a substrate 101, a first dielectric layer 110 over thesubstrate 101, a via opening 112 formed in the first dielectric layer110. In yet some embodiments, the precursor substrate 200 furtherincludes a barrier layer 120 on the first dielectric layer 110 andlining the via opening 112. In yet some embodiments, the conductivelayer 130 may fill the via opening 112 and cover the first dielectriclayer 110. The portion of the first dielectric layer 110 filled in thevia opening 112 constitutes a via contact 134 in the first dielectriclayer 110 according to some embodiments of the present disclosure.

Referring to FIG. 1B, the method 20 proceeds to operation 22 byselectively etching the conductive layer to form a first conductiveportion, a second conductive portion, and a spacing between the firstconductive portion and the second conductive portion. Referring to FIG.9B, the conductive layer 130 is selectively etched to form a firstconductive portion 131, a second conductive portion 132, and a spacing135. The spacing 135 between the first conductive portion 131 and thesecond conductive portion 132. In some embodiments, the spacing 135includes an inlet 135 i and a bottom 135 b, and the width of the inletis greater than a width of the bottom.

After operation 22, the method 20 may optionally includes an act ofremoving a portion 116 a of the precursor substrate between the firstconductive portion 131 and the second conductive portion 132 accordingto some embodiments of the present disclosure. The resulting structureis shown in FIG. 10. The removal of the portion 116 a forms an aperture137 under the spacing 135. The spacing 135 and the aperture 137collectively constitutes a void region 136.

Referring to FIG. 11, the method 20 may optionally includes an act offorming a metal capping layer 138 covering the first and secondconductive portions 131, 132 after the formation of the aperture 137.However, in yet some embodiments, the metal capping layer 138 may beformed prior to the formation of the aperture 137. For example, thecapping layer 138 may be formed immediately after the formation of thefirst conductive portion 131 and the second conductive portion 132depicted in FIG. 9B.

Referring to FIG. 1B, the method 20 proceeds to operation 23 by forminga dielectric layer over the first conductive portion, the secondconductive portion, and the spacing, such that an air gap is formed inthe spacing between the first and second conductive portions and issealed by the dielectric layer. FIG. 12A illustrates the formation of adielectric layer 140. The dielectric layer 140 is over the firstconductive portion 131, the second conductive portion 132, and thespacing 135. An air gap 142 is formed in the spacing or the void region136 under the dielectric layer 140 between the first and secondconductive portions 131, 132. The spacing or the void region 136 is notfilled with the dielectric layer 140 so to form the air gap 142 underthe dielectric layer 140. The air gap 142 is sealed by the dielectriclayer 140.

According to another aspect of the present disclosure, a semiconductorstructure is provided. FIG. 12A illustrates a cross-sectional view of asemiconductor structure 300 according to various embodiments of thepresent disclosure. FIG. 12B is a top view of FIG. 12A according to someembodiments of the present disclosure. FIG. 12A is approximately takenalong line A-A′ in FIG. 12B. As shown in FIGS. 12A and 12B, thesemiconductor structure 300 includes a first dielectric layer 110, afirst conductive wiring 131, a second conductive wiring 132, a seconddielectric layer 140 and an air gap 142 (also referred to as void gap142).

The first conductive wiring 131 and second conductive wiring 132 extendover the first dielectric layer 110. The first conductive wiring 131 andthe second conductive wiring 132 respectively have a first sidewall 131s and a second sidewall 132 s. The second sidewall 132 s is opposite tothe first sidewall 131 s. The first sidewall 131 s and the secondsidewall 132 s form an acute angle θ. In some embodiments, the acuteangle θ may be ranged from about 5 degrees to about 40 degrees, forexample about 7 degrees, about 12 degrees, about 15 degrees, about 20degrees, and about 30 degrees. A spacing is presented between the firstsidewall 131 s and the second sidewall 132 s.

The second dielectric layer 140 is disposed over the first conductivewiring 131 and the second conductive wiring 132. The second dielectriclayer 140 seals the spacing between the first and second sidewalls 131s, 132 s. The second dielectric layer 140 does not fully fill thespacing between the first and second sidewalls 131 s, 132 s. An air gap142, therefore, is formed under the second dielectric layer 140 andbetween the first and second sidewalls 131 s, 132 s.

In some embodiments, as shown in FIG. 12B, the second conductive wiring132 may extend in an extension direction D1. The second conductivewiring 132 is at least partially aligned with the first conductivewiring 131 in a perpendicular direction D2, which is orthogonal to theextension direction D1. In some examples, the second conductive wiring132 has a section 132 a aligned with the first conductive wiring 131.The length L1 of the air gap 142 along the extension direction D1 issubstantially equal to the length L2 of the section 132 a of the secondconductive wiring 132. The term “substantially equal to” herein refersto that the difference between the length L1 and the length L2 is lessthan or equal to triple width W1 of the air gap 142, in which the widthW1 is the maximum width of the air gap 142. In some examples, the lengthL1 and the length L2 satisfy the following formula (I):

|L1−L2|<(3×W1)  formula (I).

In yet some examples, the length L1 and the length L2 satisfy thefollowing formula (II):

|L1−L2|<(2×W1)  formula (II).

In yet some examples, the length L1 and the length L2 satisfy thefollowing formula (III):

|L1−L2|<(W1)  formula (III).

In yet some embodiments, a third conductive wiring 133 is included inthe semiconductor structure 300, as shown in FIG. 12B. The firstconductive wiring 131 extends in a direction D1, and has a section 131 aaligned with the third conductive wiring 133 in a perpendiculardirection D2. An air gap 143 is existed between the conductive wiring131 and the third conductive wiring 133. The length L3 of the air gap143 along the direction D1 is substantially equal to the length L3 ofthe section 131 a of the first conductive wiring 131. In some examples,the air gap 143 and the air gap 142 are arranged at opposite sides ofthe first conductive wiring 131. Furthermore, the length L3 of the airgap 143 is different from the length L1 a of the air gap 142.

In yet some embodiments, the air gap 142 includes an upper portion135(indicated in FIG. 10), a bottom portion 137, and a neck portion 136n there between, the neck portion 136 n has a minimum width W4 from theupper portion 135 to the bottom portion 137. In some examples, the upperportion 135 has a maximum width W5, and a ratio of the minimum width W4to the minimum width W5 is ranged from about 0.5 to about 0.95. In someexamples, the bottom portion 137 of the air gap 142 extends into thefirst dielectric 110.

In yet some embodiments, one or more via contacts 134 may be included inthe semiconductor structure 300. The via contacts 134 is formed orembedded in the first dielectric layer 110. The via contact 134 isconnected to at least one of the first and second conductive wirings131, 132.

In accordance with one aspect of some embodiments, a method includes thefollowing operations: forming a conductive layer over a precursorsubstrate; selectively etching the conductive layer to form a firstconductive portion, a second conductive portion, and a spacing betweenthe first conductive portion and the second conductive portion; andforming a dielectric layer over the first conductive portion, the secondconductive portion, and the spacing, such that an air gap is formed inthe spacing between the first and second conductive portions and issealed by the dielectric layer.

In accordance with another aspect of some embodiments, a method offorming a semiconductor structure includes the following operations:forming a first dielectric layer over a substrate; forming a via openingin the first dielectric layer; forming a barrier layer lining the viaopening and covering the first dielectric layer; forming a conductivelayer overlaying the barrier layer and filling the via opening;patterning the conductive layer and the barrier layer to form a firstconductive portion, a second conductive portion, and a spacing therebetween; and forming a second dielectric layer covering the spacing andthe first and second conductive portions to enclose a void gap in thespacing.

In accordance with another aspect of some embodiments, a semiconductorstructure includes a first dielectric, a first conductive wiring, asecond conductive wiring, and a second dielectric layer. The firstconductive wiring extends over the first dielectric layer and has afirst sidewall. The second conductive wiring extends over the firstdielectric layer and has a second sidewall opposite to the firstsidewall. The second dielectric layer is over the first conductivewiring and the second conductive wiring. The air gap is under the seconddielectric layer between the first sidewall and the second sidewall. Thefirst sidewall and the second sidewall form an acute angle.

1. A method of forming an interconnect structure, comprising: forming aconductive layer over a substrate; selectively etching the conductivelayer to form a first conductive portion, a second conductive portion,and a spacing between the first conductive portion and the secondconductive portion; removing a first portion of the substrate under thespacing, while remaining a second portion of the substrate under thefirst conductive portion, such that a sidewall of the second portion ofthe substrate has a different shape from a sidewall of the firstconductive portion; and forming a dielectric layer over the firstconductive portion, the second conductive portion, and the spacing, suchthat an air gap is formed in the spacing between the first and secondconductive portions and is sealed by the dielectric layer.
 2. The methodaccording to claim 1, further comprising forming a metal capping layercovering the first and second conductive portions after selectivelyetching the conductive layer.
 3. (canceled)
 4. The method according toclaim 1, wherein the spacing comprises an inlet and a bottom, and awidth of the inlet is greater than a width of the bottom. 5-20.(canceled)
 21. A method of forming a semiconductor structure,comprising: forming a first dielectric layer over a substrate; forming avia opening in the first dielectric layer; overfilling the via openingwith a conductive layer; etching the conductive layer to form separatedfirst and second conductive portions; and forming a second dielectriclayer over the separated first and second conductive portions such thatan air gap is formed under the second dielectric layer and between theseparated first and second conductive portions, and such that the airgap extends to higher elevation than a top of the first conductiveportion.
 22. The method of claim 21, wherein the forming the seconddielectric layer comprises spin-on coating a dielectric material on thefirst dielectric layer and the separated first and second conductiveportions.
 23. The method of claim 22, wherein the spin-on coating isperformed such that the dielectric material is formed on a sidewall ofthe first conductive portion farthest away from the second conductiveportion.
 24. The method of claim 21, wherein the etching the conductivelayer stops until reaching the first dielectric layer.
 25. The method ofclaim 21, wherein the forming the first dielectric layer comprisesforming a low-k dielectric layer over the substrate and forming an etchstop layer over the low-k dielectric layer, wherein the etching theconductive layer stops until reaching the etch stop layer.
 26. Themethod of claim 25, wherein a portion of the etch stop layer is exposedafter the etching the conductive layer, and the method further comprisesremoving the exposed portion of the etch stop layer such that a portionof the low-k dielectric layer is exposed.
 27. The method of claim 26,wherein the forming of the second dielectric layer comprisesspin-coating a dielectric material on the exposed portion of the low-kdielectric layer.
 28. The method of claim 21, wherein an etchant used inthe etching the conductive layer comprises at least one of Cl2, F2, H2,and CxFy.
 29. The method of claim 21, wherein the etching the conductivelayer is a reactive ion etching (ME) process using at least one of Cl2,F2, H2, and CxFy as an etchant.
 30. The method of claim 21, furthercomprising performing a chemical mechanical polishing (CMP) process tothe conductive layer prior to the etching the conductive layer.
 31. Themethod of claim 30, wherein the CMP process stops at a position higherthan a top surface of the first dielectric layer.
 32. A method offorming a semiconductor structure, comprising: forming a firstdielectric layer over a substrate; forming a via opening in the firstdielectric layer; forming a conductive layer in the via opening and overthe first dielectric layer; forming an opening in the conductive layerto expose a portion of the first dielectric layer; and forming a seconddielectric layer over the conductive layer by a spin-on coating process,such that the second dielectric layer over the opening of the conductivelayer is separated from the exposed portion of the first dielectriclayer by an air gap having a top in a position higher than a top of theconductive layer.
 33. The method of claim 32, wherein the forming thefirst dielectric layer comprises forming in sequence a low-k dielectriclayer and an etch stop layer over the substrate, wherein the methodfurther comprises performing an etching process to the exposed portionof the first dielectric layer prior to the forming the second dielectriclayer, such that the etch stop layer in the exposed portion is removed.34. The method of claim 33, further comprising forming a barrier layerin the via opening and over the first dielectric layer prior to theforming the conductive layer, wherein the conductive layer is formedover the barrier layer, and the forming the opening in the conductivelayer further forms an opening in the barrier layer to expose theportion of the first dielectric layer.
 35. The method of claim 32,further comprising forming a capping layer over the conductive layerprior to the forming the second dielectric layer, wherein the seconddielectric layer is formed over the capping layer.
 36. The method ofclaim 32, further comprising performing a chemical mechanical polishing(CMP) process to the conductive layer prior to the forming the openingin the conductive layer, wherein the CMP process stops before reachingthe first dielectric layer.
 37. The method of claim 1, wherein theremoving the first portion of the substrate is such that the sidewall ofthe remaining second portion of the substrate is curved.